1. Field of the Invention
This invention relates to a heavy duty pneumatic tire, and particularly to improvement of a rubber composition for a tread of the pneumatic tire mounted on a large vehicle which runs on rough roads (so called, an off-the-road tire).
2. Description of Related Art
Up to now, for a heavy duty pneumatic tire for a vehicle running on rough roads attempts has been made to improve low heat build-up, wear resistance, appearance and the like.
In order to improve a tread rubber composition, there are concretely various techniques of selecting the kind of carbon black compounded therein, adjusting the amount of the carbon black and the like.
In general, a predetermined amount of the carbon in the tread rubber composition can improve the tire in the wear resistance and the cutting resistance due to an increased rubber hardness, so that the tire obtains a long life. However, too much carbon black compounded therein lowers not only the workability but also the elongation at the breaking (Eb), so that the cutting resistance and the chipping resistance during running are degraded with consequence of lowering the appearance. Additionally, a large amount of carbon black compounded causes failures such as the heat separation due to high heat build-up and the like.
Further, there is a technique of compounding a resin in the tread rubber composition. The added resin raises the elongation at breaking (Eb) and can improve the appearance, but lowers the wear resistance.
Moreover, there is a technique of blending styrene butadiene rubber (SBR) having a high hardness at a low deformation into natural rubber (NR) having the large elongation at breaking at high temperatures. But the technique has a problem of the heat build-up.
Further, for the purpose of the depress the heat build-up, compounding a solution polymerized SBR which has the low heat build-up brings the low breaking resistance to degrade the appearance in running on rough roads. Therefore, for the purpose of solving the above problem, there is a technique of combining a specific carbon black and the solution polymerized SBR to improve the appearance while keeping the wear resistance and the low heat build-up (see: JP-A-5-117450).
There is, recently, developed a technique of compounding a specific carbon black and silica with isoprene based rubber to make compatible the low heat build-up and the wear resistance without detracting the workability (JP-A-9-268237).
However, a demanded level of improvement is not satisfied.
It is, therefore, an object according to the invention to improve the appearance, wear resistance, and low heat build-up in a heavy duty pneumatic tire at a higher level.
In order to attain the above object, the heavy duty pneumatic tire according to the invention has the following construction.
A heavy duty pneumatic tire according to the invention is characterized in that a tread rubber of the tire is constructed with a rubber composition comprising 20-67 parts by weight of carbon black and 3-20 parts by weight of silica based on 100 parts by weight of diene based rubber, wherein the carbon black has a ratio of a dibutyl phthalate (DBP) absorption to a 24M4 DBP absorption (DBP absorption/24M4 DBP absorption) of not less than 1.20, and the ratio of the DBP absorption to a nitrogen adsorption specific area (N2SA) of not less than 0.8.
In the above construction, the carbon black preferably has the N2SA of 120-160 m2/g and the DBP absorption of 120-150 ml/100 g, and the silica preferably has a N2SA of 210-260 m2/g and a DBP absorption of 200-260 ml/100 g. Further, the carbon black preferably has the ratio of the DBP absorption to the 24M4 DBP absorption (DBP absorption/24M4 DBP absorption) of not more than 1.30, and the ratio of the DBP absorption to the N2SA of 0.82-2.00.
The invention will be described in detail below.
In a tread rubber composition of the heavy duty pneumatic tire according to the invention, the rubber ingredient is defined to be diene based rubber because of a high breaking resistance and a good balance of the low heat build-up and the wear resistance.
Further, diene based rubber preferably usable in the invention is isoprene based rubber. Concretely, at least one selected from the group consisting of natural rubber and isoprene rubber (IR) is preferably of not less than 50% by weight, more preferably not less than 80% by weight, particularly preferably 100% by weight.
With respect to the carbon black according to the invention, the ratio of DBP absorption/24M4 DBP absorption is defined to be not less than 1.20. Because when the ratio is less than 1.20, an elongation of the vulcanized rubber composition is small and hence the appearance is degraded owing to generation of cutting or chipping. Besides, the term xe2x80x9c24M4DBP absorptionxe2x80x9d used in the invention means a DBP absorption after compressing the carbon black four times at 24.000 p.s.i. according to ASTM D3493, and it is adopted for evaluation of primary structure which is not destroyed by mixing etc.
Further, the ratio of DBP absorption/24M4 DBP absorption is preferably not more than 1.30. Because, when the ratio exceeds 1.30, the low heat built-up tends to be degraded.
Further, with respect to the carbon black in the invention, the ratio of DBP absorption/N2SA is defined to be not less than 0.8, which can bring the desired wear resistance. When the ratio is less than 0.8, wear resistance is not sufficiently improved. The ratio is preferably not more than 1.25, which can bring the larger elongation of the rubber and the improved appearance. Further, when the ratio exceeds 2.00, an elongation of the rubber is not sufficiently improved.
Moreover, the carbon black preferably has the N2SA of 120-160 m2/g and the DBP absorption of 120-150 ml/100 g. Because when the N2SA is less than 120 m2/g or the DBP absorption is less than 120 ml/100 g, the improvement effect of the wear resistance brought by combination of the carbon black and the silica is not sufficient; while when the N2SA exceeds 160 m2/g or the DBP absorption exceeds 150 ml/100 g, the low heat build-up is degraded, and the balance of the wear resistance and the low heat build-up is not improved sufficiently. From the same point of view as in the above, the carbon black preferably has the N2SA of 130-150 m2/g and the DBP absorption of 125-140 ml/100 g.
In the invention, the amount of the carbon black compounded is defined to be 20-67 parts by weight based on 100 parts by weight of the diene based rubber. Because, when it is less than 20 parts by weight, wear resistance is not sufficient, while when it exceeds 67 parts by weight, the low heat build-up is degraded.
In the invention, the silica preferably has the N2SA of 210-260 m2/g and the DBP absorption of 200-260 ml/100 g. Because the combination of the silica and the carbon black having the properties within the above ranges can bring an optimum dispersion state of the carbon black and the silica, which can make compatible the low heat build-up and the wear resistance. Concretely, when the N2SA exceeds 260 m2/g, a viscosity of the vulcanized rubber composition rises to decrease workability in manufacturing the tire. When it is less than 210 m2/g, the wear resistance is not improved sufficiently. Further, when the DBP absorption exceeds 260 cc/100 g, the low heat build-up is not improved sufficiently. When it is less than 200 ml/100 g, the good balance of the wear resistance and the low heat build-up is difficult to be kept. From the same point of view as in the above, silica more preferably has the N2SA of 210-240 m2/g and the DBP absorption of 220-240 ml/100 g.
Further, the amount of the compounded silica is defined to be 3-20 parts by weight, preferably 5-15 parts by weight based on 100 parts by weight of the diene based rubber. Because, when it is not in this range, the good balance of the wear resistance and the low heat build-up is difficult to be kept.
In the specification and claims of the invention, the N2SA of the carbon black is measured according to ASTM D4820-93, the DBP absorption thereof ASTM D2414-93, and the 24M4 DBP absorption thereof ASTM D3493; the N2SA and the DBP absorption of the silica are measured according to the same ways as in the above after drying it at 300xc2x0 C. for one hour.
Besides, in the invention, in addition to the above ingredients, vulcanizing agent, vulcanization accelerator, vulcanization aids and other additives, which are usually used, may be compounded in the proper amounts.